feat(core/fetcher): make graffiti configurable per validator

[crStiv]

Replace hardcoded graffiti with configurable function in Fetcher struct.
This allows operators to set custom graffiti for each validator while
maintaining the default version+commit behavior.

Changes:

• Add graffitiFunc field to Fetcher struct
• Add SetGraffitiFunc method
• Move default graffiti logic to defaultGraffiti function

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud